88 ON AGE AND AFPEABAXCE. 



the tushes are generally absent, and sometimes there are 

 a few supplementary teeth. 



The teeth are placed in very deep sockets in the jaw- 

 hones, by which, with the assistance of the gums, they are 

 fii'mly kept in their situation. 



The structure of the teeth is very curious. In the horse 

 we find it composed of three portions — first, the bone, or 

 rather the ivory, for it is similar to the tusks of the ele- 

 phant, forms the bulk of the tooth ; secondly, the enamel, 

 which is exceedingly hard, and forms the surface of the 

 crown and a portion of the face, and dips into the body of 

 the tooth ; thirdly, the crusta petrosa, softer than the 

 other portions, and more opaque. This portion appears to 

 be a sort of cement to unite the other constituents together, 

 but it is only found in herbivorous animals. 



In the foetus of three or four months old, we find the 

 germ of the first pair of teeth in the alveolar cavity; it ap- 

 pears a soft pulpy substance secreted by a membranous 

 capsule, which in an incisor tooth is single, but in the 

 molar there are no less than four to the under, and five to 

 the upper, which accounts for the irregular appearance of 

 the molar teeth. The pulp is gradually changed into the 

 bard material. The membrane of the incisor teeth that 

 secreted the pulp is double, and from its outer surface it 

 afterwards secretes the enamel, and from its inner the 

 ivory. A tooth is divided into crown and fang, the former 

 being that portion outside the gum, and the latter that 

 contained within the socket ; whilst the part immediately 

 embraced by the gum is called the neck. The upper sur- 

 face of the crown is called the face, and is that part on 

 which the mark is situated in the incisor, by which we 

 judge of the age. 



Before the age of five, the age of the horse is ascertained 



